Programmed cell death is a fundamental biological process required for the development and maintenance of tissue homeostasis. Deregulation of apoptosis can result in excessive cellular demise such as in cytopenias (e.g. thrombocytopenia) or pathologic cellular survival as seen in hematologic cancers (e.g. follicular lymphoma). Cellular life and death is regulated by an expansive family of BCL-2 proteins, which comprise an intricate network of guardians and executioner proteins. The multi-domain pro-apoptotic members BAX and BAK serve as the ultimate gatekeepers of cell death and, once activated, irreversibly induces cellular demise. Despite BAX and BAK's essential role in apoptosis, double knockout mice develop normal thoracic and abdominal organs with moderate excess accumulations of hematopoietic cells. These data suggest that additional pro-apoptotic multidomain proteins may subserve the functions of BAX and BAK in such tissues. Chief among the candidates for a central role in the execution of mitochondrial apoptosis is BOK, a widely- expressed close homologue of BAX and BAK. I propose a multidisciplinary approach that employs synthetic chemistry techniques, biochemical and cellular experiments, and mouse modeling to investigate the physiologic role of BOK in maintaining tissue homeostasis, with a focus on hematopoiesis. I aim to: (1) Design, synthesize, and purify recombinant BOK protein and stapled peptide modulators to investigate BH3- triggered BOK activation; (2) Define the functional activity of ligand-induced BOK oligomerization; and (3) Determine the physiologic role of BOK-mediated apoptosis in the development and homeostasis of the hematopoietic system. Stabilized Alpha-Helices of BCL-2 domains (SAHBs) that retain the specificity of native BH3 death ligands and directly activate BOK will serve as unique molecular probes and prototype therapeutics to investigate and modulate BCL-2 family interactions in vitro and in vivo. Thus, the proposed mechanistic dissection of BOK activity will advance our goal of developing novel pharmacologic approaches to regulate BOK for therapeutic benefit in blood diseases of pathologic cell survival or premature cell death. The research plan will be executed in the context of a comprehensive career development plan that includes courses, meetings, workshops, a rich academic environment, a scientific advisory board with deep expertise, and an accomplished multidisciplinary physician scientist mentor. These components will propel the project to successful completion and advance the investigator's independent career as a hematopathologist specializing in hematopoietic apoptosis biology and its clinical applications.